A diesel-engine exhaust system may include a diesel particulate filter (DPF) to limit particulate emissions. The DPF may include a cylindrical array of cells made of a porous material. Exhaust gas may be flowed through the DPF so that particulate matter entrained in the exhaust stream—soot, for example—is separated from the exhaust stream and collected within the cells.
As a DPF accumulates particulate matter, its capacity for continued exhaust filtration decreases. Therefore, a DPF at reduced capacity may be subject to a restoration phase, wherein high-temperature engine exhaust, hydrocarbon-rich engine exhaust, and/or nitrogen-dioxide rich engine exhaust—as examples—are provided to the DPF inlet. Soot trapped within the cells of the DPF may thereby be oxidized to volatile products, which flow from the DPF outlet in the filtered exhaust stream. The capacity of the DPF for subsequent filtration is thereby restored.
Oxidation of soot in the DPF is exothermic, however, and an uneven distribution of soot within the DPF may result in corresponding, uneven temperature gradients there within. Soot may deposit unevenly as a result of the fluid-flow pattern through the array of DPF cells; it may accumulate more heavily near the major axis and to the outlet side of the array, for example. When the temperature gradients in the DPF are not held within acceptable limits, inhomogeneous thermal expansion of DPF structures may lead to cracking and/or structural degradation, such as due to radial and longitudinal stresses related to the thermal temperature gradients across the structure.
Therefore, various approaches have been taken to lessen the temperature gradients experienced within a DPF during regeneration. For example, U.S. Patent Application Publication Number 2003/0097834 describes a DPF in which some cells near the major axis are closed off to limit exhaust flow and deposition of soot near the major axis. This approach attempts to lessen a radial component of the temperature gradient (i.e. the component from the major axis of the cell array to the exterior), but may not address a longitudinal component of the temperature gradient, i.e., from the outlet side of the array to the inlet side. This approach may also limit the capacity of the DPF by closing off cells throughout the entire length of the particular filtering material that may otherwise be used to trap exhaust-stream particulates.
The inventors herein have recognized these limitations and have devised a series of approaches to address them. Thus, in one embodiment, a DPF configured to separate a particulate from an engine exhaust is provided. The DPF comprises a downstream filtration stage including a plurality of downstream channels, an upstream filtration stage disposed upstream of and in fluidic communication with the downstream filtration stage, and including a plurality of upstream channels, and a shell at least partly enclosing the downstream filtration stage and the upstream filtration stage, the shell including an inlet configured to conduct the engine exhaust to the upstream filtration stage and an outlet configured to release the engine exhaust from the downstream filtration stage. In one embodiment, the plurality of upstream channels includes a plurality of open upstream channels, the arrangement of which disperses the particulate more evenly over the plurality of downstream channels, as more flow is directed away from the major axis of the filter. In this way, during regeneration, it is possible to reduce one or both of longitudinal and radial thermal gradients, and thereby improve structural integrity of the filter.
Other embodiments provide an exhaust aftertreatment system comprising a DPF and a diesel-oxidation catalyst module, and a method to restore a capacity of a DPF.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the Detailed Description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the Detailed Description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.